26o 



BIOLOGY OF THE LABORATORY MOUSE 



incidence of breast cancer was somewhat reduced as compared with normal 

 breeding females. 



All of this suggests that the cyclic changes to which the breast tissues are 

 subjected in pregnancy and lactation are periods during which the risk of 

 setting up a neoplastic process is very definitely increased. 



Evidence that coat color may play a part in influencing the incidence of 

 mammary tumors. — It will be well at the outset to make clear the fact that 

 various degrees of incidence of breast tumors exist in distinct inbred strains 

 of different coat colors. This, however, does not necessarily mean that coat 

 color per se affects the incidence directly or even that it represents a general 

 physiological type which is more or less susceptible. Selection which estab- 

 lishes any characteristic coat color for a given inbred strain may also fix the 



Table 9 



tumor incidence coincident with but entirely independent of the question of 

 color. 



The real test of the influence of coat color depends upon the comparison 

 of different colored animals within the same generation of mice, where other 

 genetic influences have been equalized as nearly as possible. 



The opportunity to study this type of situation is offered by comparing 

 yellow and non-yellow mice among the animals of Fi and Fo generations 

 following an outcross. 



Since all yellows so far observed are heterozygous,, being ^^a or A'-' A in 

 formula, the Fi generation of a cross with aa (non-yellow) mice consists of 

 yellows and non-yellows in approximately equal numbers. 



The F2 generation gives yellows and non-yellows in proportions which 

 var>' according to the color of the Fi animals selected for breeding. 



In a cross reported by Little (54) the figures shown in Table 9 were 

 obtained. 



